When someone is in the middle completing something but they get utterly obliterated. It is an onomatopoeia for the property that slush has when you step on it.
According to Donald Trump, Hillary Clinton got schlonged by Barack Obama in the 2008 democratic presidential primary. This is akin to getting screwed; slighted; severely whooped; underhandedly beaten; having your ass handed to you ,etc.
She (Hillary) was gonna beat (Obama), she was favored to win (the 2008 Democraticpresidential primary), and she got schlonged, she lost.